1. Field of the Invention
The present invention relates to signal correlators for digital signal receivers and, more particularly, the invention relates to a method and apparatus for performing frequency synchronization.
2. Description of the Background Art
The process of measuring a Global Navigation Satellite System (GNSS) signal begins with a procedure to search for the GNSS signal in the presence of noise by attempting a series of correlations of the incoming signal against a known pseudo-random noise (PRN) code. The search process can be lengthy, as both the exact frequency of the signal and the time-of-arrival delay are unknown. To find the signal, receivers traditionally conduct a two dimensional search, checking each delay possibility at every possible frequency. To test for the presence of a signal at a particular frequency and delay, the receiver is tuned to the frequency, and the incoming signal is correlated with the known PRN code delayed by an amount corresponding to the time of arrival. If no signal is detected, the search continues to the next delay possibility, and after all delay possibilities are checked, continues to the next frequency possibility. Each individual correlation is performed over one or more milliseconds in order to allow sufficient signal averaging to distinguish the signal from the noise. Because many thousand frequency and delay possibilities are checked, the overall acquisition process can take as much as tens of seconds.
Recently, new applications of GNSS technology in wireless devices have emerged, for example, the use of GNSS in cellular phones to provide emergency location capability. In these applications, rapid signal acquisition in just a few seconds is required. Furthermore, these applications require a GNSS receiver to operate in harsh signal environments and indoors, where GNSS signal levels are greatly attenuated. Detecting attenuated signals requires each correlation to be performed over a relatively long period of time. For example integration may be performed over a few seconds, as opposed to the 1-10 millisecond period used in traditional GNSS receivers. The two-dimensional sequential search process employed by traditional receivers to synchronize time and frequency values becomes impractical at such long integration times, because the overall search time increases by a factor of 100 or more.
To accelerate the search process, GNSS designers add additional correlators to the receiver so that multiple time of arrival possibilities can be tested simultaneously. Typically, each correlator that is added requires a separate code mixer and signal accumulator. For a given sensitivity level, this decreases search times in proportion to the number of correlators. To achieve the sensitivity and acquisition time demanded in cellular phone applications, the design might have to incorporate thousands of correlators. Various techniques, such as multiplexed use of correlators, have been developed to facilitate the functionality of many correlators without having the physical correlators in the receiver. Code delay tracking is typically performed by searching all possible code delays, finding a best matching delay, then establishing a “window” about the delay in which correlations are generated. The code is adjusted to maximize the correlation output.
To properly correlate a signal, the receiver must track both the code delay and frequency of the GNSS signal. Adding numerous parallel correlators reduces the time used to search for signal delays, but does not reduce the search time used to achieve frequency lock.
Frequency tracking is typically performed using a conventional phase or frequency lock loop. Once a frequency lock is achieved through an exhaustive search of all possible frequencies, the phase or frequency lock loop maintains the local oscillator at a frequency that optimizes the correlator output.
However, such exhaustive delay and frequency searches are time consuming and, once synchronization is received, the tracking techniques do not operate very well at low signal levels. Such low signal levels (−148 dBm to −160 dBm) are common during reception of GNSS satellite signals.
Therefore, there is a need in the art for an improved technique for performing frequency search and tracking.